Field of the Invention
The invention relates to a method for the coating of surfaces of TiAl alloys, in which at least one layer is electroplated on the surface.
Prior Art
In turbomachines such as stationary gas turbines or aircraft engines, in order to increase the efficiency of the turbomachines, TiAl alloys are increasingly used, which make possible a more efficient operation of the turbomachine with simultaneously high strength due to their low specific gravity. Of course, prevailing in turbomachines are ambient conditions that require the introduction of additional protective layers, such as layers for protection against erosion, layers for protection against oxidation, heat insulating layers, and the like.
Frequently, an electroplated metal layer is provided between the component surface and the coating as a base layer or intermediate layer, in order to introduce these types of protective layers.
Similar to the case of titanium alloys and aluminum alloys, which very rapidly form oxide layers due to the affinity of their principal alloy components, titanium and aluminum, for oxygen, TiAl alloys also frequently very rapidly form an oxide layer on the surface due to the principal components, titanium and aluminum, and this makes difficult or even impossible an electroplating of a metal layer.
However, in order to make possible an electroplating of a metal layer on a surface containing titanium and/or aluminum, it is already known to roughen the surface in order to facilitate or to make possible the electroplating by means of the formation of projecting sharp points on the surface. Of course, the known methods employing a mechanical roughening or a chemical etching of the surface are not satisfactory, since either the methods are expensive or they lead to unsatisfactory results.
In the case of mechanical surface roughening, unwanted deformations and damage of the surface region can occur, and other methods, such as chemical methods, frequently do not supply the necessary adhesive strength or roughness of the surface for the subsequent electroplating.